Surge arrester monitoring theory
6.3 Leakage current measurements
Apart from the brief occasions when a surge arrester is functioning as an overvoltage-limiting device, it is expected to behave like an insulator. The insulating properties are essential for the length of life of the arrester and for the operation reliability of the power system.
Any deterioration of the insulating properties of a metal-oxide arrester will cause an increase in the resistive leakage current, at given values of voltage and temperature. Therefore, the resistive leakage current in service can be used as a diagnostic tool to check the condition of a surge arrester. Leakage current measurements for diagnostic purposes are usually made on temporary basis at regular intervals.
Repeated measurements may be necessary for closer investigations, if significant changes in the condition of an arrester are revealed by temporary measurements.
Leakage current of metal-oxide arresters
The total leakage current of a metal-oxide arrester can be divided into capacitive and resistive parts, see figure 6.3.1, with a predominant capacitive component and a significantly smaller resistive part (5 to 20% of the total current). The capacitive leakage current is caused by the permittivity of the metal-oxide varistors, by the stray capacitances and by internal grading capacitors (if applied).
A large increase in the resistive leakage current is needed before a noticeable change occurs in the total leakage current level. Therefore, the total leakage current is unsuitable for arrester diagnostic purposes.
Instead, it may be used for other diagnostic or maintenance purposes, e.g. to estimate the prevailing amount of insulator surface pollution and the associated need for insulator washing, greasing etc. of the insulators in the substation.
The resistive component of the leakage current, on the other hand, is a sensitive indicator of any changes in the voltage-current cha- racteristic of a metal-oxide arrester. The EXCOUNT-II is equipped for measurement of the total leakage current and, optionally, for measure- ment of the resistive leakage current. In this way, the EXCOUNT-II may be equipped to fulfil different diagnostic needs in addition to surge counting.
6. Surge arrester monitoring theory
Figure 6.3.1
Electrical representation of metal-oxide surge arrester in the leakage current region Resistive leakage current
The resistive leakage current is defined as the peak value of the resis- tive component of the leakage current, i.e. the instantaneous value of the leakage current when the voltage across the arrester is at its maxi- mum (dU/dt = 0). In the leakage current region, the resistive current depends on the voltage stress and temperature of the varistors. The typical non-linear behavior of the resistive leakage current is shown in figure 6.3.2 for two different varistor temperatures. The voltage stress is expressed as the ratio of the operating voltage to the rated voltage of the arrester (U/Ur).
Figure 6.3.2
Example of voltage-current characteristics of a metal-oxide surge arrester
The maximum continuous operating voltage of an arrester (the Uc according to IEC, or the MCOV according to ANSI) usually cor- responds to a voltage stress in the range 0,7-0,85 p.u. of the rated voltage.
In normal arrester applications, the operating voltage stress usually ranges from 0,5 to 0,8 p.u. of the rated voltage. In this range, the resistive leakage currents at +20°C may vary from 10 to 600 µA depending on the size and make of the varistors.
Harmonics in the total leakage current
The non-linear voltage-current characteristic of a metal-oxide arrester, illustrated in figure 6.3.2, gives rise to harmonics in the total leakage current when the arrester is energized with a sinusoidal voltage. The harmonic content depends on the degree of non-linearity, which is a function of voltage stress, temperature and make of the arrester. As an example, the third harmonic content of the total leakage current is typically 10-40% of the resistive current.
The harmonic content of the total leakage current can, therefore, be used as an indicator of the resistive leakage current. Using harmonics for measuring the resistive leakage current is advantageous compared to other methods, since no voltage reference is needed to determine the resistive part of the total leakage current. The third order harmonic is of special interest in this respect, since it has the largest magnitude of the current harmonics.
The actual resistive leakage current level can be readily determined from measurements of the third harmonic, provided the appropriate information is available regarding the third harmonic content of the resistive current at the prevailing voltage stress and temperature. This information is specific to the arrester make and type, and must there- fore be supplied by the arrester manufacturer.
6. Surge arrester monitoring theory
6. Surge arrester monitoring theory Another source of harmonics in the total leakage current is the har-
monic content in the system voltage. The voltage harmonics produce capacitive harmonic currents in the arrester. This is clearly illustrated in figure 6.3.3, showing results from total leakage current measurements on two different arresters in service conditions that are significantly different in terms of system voltage harmonics.
Figure 6.3.3
Total leakage currents of metal-oxide surge arresters in different service conditions
The capacitive harmonic currents produced by the voltage harmonics may be of the same order of magnitude as the harmonic currents generated by the non-linear resistive leakage current. This means that the third harmonic content originating from the system voltage interferes with the third harmonic content associated with the resistive leakage current of the arrester. In order to perform accurate measure- ments of the resistive leakage current by means of third order harmo- nic analysis, it is therefore necessary to compensate for the third order harmonic content in the system voltage.
Leakage current measurements with EXCOUNT-II When the EXCOUNT-II is optionally equipped for measurement of resistive leakage currents, the measurement is based on third har- monic analysis of the total leakage current with compensation for the third harmonic in the system voltage. The compensation is performed by simultaneous measurements of both the total leakage current of the arrester and the current induced in a field probe, the latter being proportional to the harmonic content in the system voltage.
The principle for measurement of the resistive leakage current with the EXCOUNT-II is the same as for the original leakage current monitor, LCM, developed by ABB Switchgear and TransiNor, and described in detail in [2].
The procedure for total leakage current and field probe current measurements with EXCOUNT-II is presented step-by-step in the fol- lowing:
Figure 6.3.4
Internal parts of the EXCOUNT-II sensor 6. Surge arrester monitoring theory
Zero-flux current
transformer Field probe
6. Surge arrester monitoring theory The total leakage current is measured by means of the zero-flux
current transformer, and the electric field generated by the system voltage is measured in terms of the current induced in the field probe, both shown in figure 6.3.4. The field probe current, see figure 6.3.5, is used to compensate for the harmonic content in the system voltage.
Every 10 seconds, the data communication system of the EXCOUNT-II sensor is activated to establish contact with an EXCOUNT-II transceiver in the vicinity. If successful, the sensor makes the total leakage current and field probe current measurements described above and transmits the measured data to the transceiver. In addition, the sensor also trans- mits surge counting data along with data on ambient temperature and sensor identity, etc. For details on the sensor/transceiver data commu- nication system, see technical data in section 11 on page 42.
Figure 6.3.5
Principle of field probe for determination of system voltage harmonics
In the transceiver, the magnitudes of the total leakage currents are checked with regard to the measurement ranges (see Table 1). Extre- mely low current levels, caused by the arrester being out of opera- tion etc., are also identified. Accepted measurements are analysed by means of Discrete Fourier Transformation (DFT) to determine the magnitude and phase angle of the first and third order harmonic components of the total leakage and field probe currents (for resistive leakage current option). Several measurements are analysed to verify the stability of the current levels. The total leakage current and field probe current data (as well as the surge counting data) are temporarily stored in the transceiver for later downloading to a personal computer.
Field probe
Electric field surrounding the arrester
Ip
By means of the EXCOUNT-II software, the total leakage, field probe current and surge counting data are analyzed and presented for each arrester. The resistive leakage current level (optional) is calcu- lated in two steps: First, the resistive third harmonic of the arrester resistive leakage current, with compensation for the third harmonic in the voltage, is determined by the equation below (for a three-phase horizontal installation). For a detailed explanation of the equation and its basis, see [2]. Secondly, the resistive leakage current is determi- ned from the resistive third harmonic current by means of information supplied by the arrester manufacturer.
The ratio of the total resistive leakage current to the third harmonic current depends on the operating voltage stress (the operating voltage divided by the rated voltage) and the arrester temperature (in practice, the ambient temperature). These parameters are therefore recorded at the time of the total leakage current and field probe current measu- rements. The ambient temperature is automatically measured by the sensor, while the operating voltage is entered into the transceiver at the time of the total leakage current and field probe measurement.
Figure 6.3.6
Resistive leakage current information from the surge arrester manufacturer
The information from the arrester manufacturer is given in accordance with IEC 60099-5 [1] for each arrester type. All ABB type arresters are included in the EXCOUNT-II software to allow measurements of resis- tive leakage currents. To be ably to correctly calculate the resistive lea- kage current for non-ABB type of arresters the characteristics of that type must be added to the data base. Please contact your ABB office for further information. The manufacturer’s information comprises:
• Maximum recommended levels of total resistive leakage current and resistive third harmonic current at a specified voltage stress (U/Ur = 0,7) and a specified ambient temperature (+20°C). These conditions are referred to as “standard operating conditions”.
6. Surge arrester monitoring theory
6. Surge arrester monitoring theory
• Multipliers for the total resistive leakage current and the resistive third harmonic as functions of voltage stress and ambient tempera- ture. These multipliers are used for converting the actual values of voltage stress and ambient temperature at the time of measurement to standard operating conditions. Examples of such multipliers are given in Figures 6.3.7 and 6.3.8.
Figure 6.3.7
Typical information for conversion to standard operating voltage conditions
Figure 6.3.8
Typical information for conversion to standard ambient temperature conditions
Evaluation of resistive leakage current levels
By means of the manufacturer information, the resistive leakage current level is determined from the resistive third harmonic current, and the results obtained under the actual operating conditions are converted to the standard operating conditions. After conversion, the results of the leakage current measurements can be evaluated in two different ways:
• The converted leakage current level can be compared with previous results obtained for the same arrester, to reveal any significant chan- ges in the leakage current level over time.
• The converted leakage current level can be compared with the maximum level recommended by the arrester manufacturer.
These comparisons are carried out by the EXCOUNT-II software. The results may be presented and documented in graphs, tables and reports.
References
[1] IEC 60099-5 Ed. 1.1 (2000-03): Surge arresters - Part 5: Selection and application recommendations.
[2} J. Lundquist, L. Stenström, A. Schei, B. Hansen, ”New Method for Measurement of the Resistive Leakage Currents of Metal-Oxide Surge Arresters in Service,” IEEE Trans. On PWRD, Vol. 5, No. 4, November 1990.
6. Surge arrester monitoring theory